| Forum Home | ||||
| Press F1 | ||||
| Thread ID: 39316 | 2003-11-02 10:41:00 | It's me, the exploding CD guy :) | JamesStewart (874) | Press F1 |
| Post ID | Timestamp | Content | User | ||
| 188755 | 2003-11-03 05:19:00 | And while Terry is looking at the theory, Atlas Copco AB (the big Swedish engineering company) have done some experiments. The report is here (www.paintbug.com), and there's even a movie (powerlabs.org). Scary. |
Graham L (2) | ||
| 188756 | 2003-11-03 05:19:00 | And while Terry is looking at the theory, Atlas Copco AB (the big Swedish engineering company) have done some experiments. The report is here (www.paintbug.com), and there's even a movie (www.powerlabs.org). Scary. |
Graham L (2) | ||
| 188757 | 2003-11-03 05:40:00 | Boooo Hooooo I get a messege saying "Forbiddon access or something" (p.s i'm in red hat linux) |
ilikelinux (1418) | ||
| 188758 | 2003-11-03 05:50:00 | Wonder what breed of dog Terry has and what is he training them for.If he had whippets like yours truly he wouldn't need to train them and he could get on with calculations;) | Thomas (1820) | ||
| 188759 | 2003-11-04 00:33:00 | Thin rotating disks are quite interesting. I first got involved with them in the earlier days of silicon wafer slicing. The need was for thinner diamond edged wheels so as to minimise the waste produced. Even so the wheels were thicker than the slices, and discs supported at the outer edge, with the cutting edge at a central hole were used for strength reasons. Discs with the diamond impregnation at the outer rim were more convenient to use, and made machine design somewhat easier. The interesting thing is the travelling and standing wave patterns that can be seen with a stroboscope when passing through modal frequencies, of which more below. Disk instability, dynamic and buckling, was the limiting factor for external slicing discs. My contribution was to stiffen thin cutting disks by enclosing them between pressurised water lubricated thrust bearings having a U shaped cut out at the outer edge to expose a small portion of the disc where the silicon crystal was cut. Ive based the calculations on the dimensions of a CD. I dont know if other types of disk are different, they could be. I also had to assume a value for Poissons Ratio, couldnt find that, so used 0.3. Also I'm not too au fait with the behaviour of plastics beyond the region of elasticity, except that they will be highly plastic :D For Polycarbonate the elongation at break is 60%, which is considerably more than steel. The principle stresses are naturally enough tangential and radial, and as the CD is rotated the maximum tensile strength stress (9500 psi) will be reached first tangentially at the perimeter of the hole. Just giving the result of calcs ( Ive based the calcs on "Roark" formulas for stress and strain , which will be familiar to mech. engineers), I wont bore anyone with maths. The estimated bursting speed for a CD based only on simple stress with the CD remaining circular comes out at about 41,000 rpm. However well before this speed is reached there will be considerable radial expansion of the CD in the elastic region followed by plastic deformation. It is not possible for this deformation to take place perfectly symmetrically as it grows in diameter and the CD will tend to deform elliptically? and go out of balance creating yet higher stresses. So the figure of about 27,000rpm measured in Grahams reference is not surprising. There is though yet more considerations, real life is quite complicated, a disc experiences transverse vibrations as well as pure radial extension. Well before the bursting speed, and in fact below the typical running speeds of 52x drives, the CD will go through its critical speed. This is too complicated to go into except to say there is a forward natural frequency and a backward natural frequency, ( Another purpose of the hydrostatic water bearings was to supress transverse vibration), and calculation is difficult. Based on calcs of the stationary natural frequency of a disk and making a guestimate, the critical frequency of a CD should be around 6500rpm. There is yet another effect on a CD, and that is the aerodynamics of a disk in an enclosure, which will be quite different to a disc in free air of infinite extent. For a disc in open air, the flow of air over the disc tends to provide a transverse stiffening effect. In an enclosure like a CD drive the air will be turbulent, spinning to a certain extent with the disc, and/or will have vigorous vortices. This may well generate aerodynamic instability, and add to stresses. The reported low fatigue strength of Polycarbonate may well mean that re-writable discs could fail after 'x'?? number of re-writes. |
Terry Porritt (14) | ||
| 188760 | 2003-11-04 00:42:00 | Oh, and I forgot Thomas, the dog is a Border Collie X, got him from the SPCA about 9 weeks ago. Why dont Whippets need training, do they go too fast to sit and stay? |
Terry Porritt (14) | ||
| 188761 | 2003-11-04 00:45:00 | pheew,I am glad that is cleared up;) | Thomas (1820) | ||
| 188762 | 2003-11-04 00:57:00 | I love the way Border collies look at you,sort of what can I do for you next. Whippets just do what they are bread to do,run and woe betide a rabbit that takes them on,poetry in motion. |
Thomas (1820) | ||
| 188763 | 2003-11-04 05:57:00 | If anyone is worried about exploding CDs, there is a utility called CD Speed that allows the maximum speed to be controlled. It works with my writer, but only gave the choice of 1x or 52x with my Liteon 52x cdrom drive, nothing in between. www.tonec.com |
Terry Porritt (14) | ||
| 1 2 3 | |||||